The complex genetic and biochemical programs that give rise to three- dimensional tree-like branching structures such as the lung, kidney and vascular systems have begun to be elucidated. The most well characterized of these branching systems is the Drosophila tracheal system and the mammalian lung, both of which require intact FGF signaling pathways for normal morphogenesis (1,2). For these branching events, the FGF signaling pathway establishes the initial pattern of branching which is subsequently modified by feedback mechanisms and other signals which result in increasingly more complex branching patterns. The evolutionary conservation of use of the FGF signaling pathway for branching morphogenesis of the fly and mammalian respiratory systems suggests a more general mechanism for branching of other organs such as the vasculature. An FGF antagonist called Sprouty (spr) was identified in a screen for mutations that affect branching of the Drosophila tracheal system (3). In flies that are null for spry, excessive tracheal branching occurs to branching of the Drosophila tracheal system (3). In flies that are null for spry, excessive tracheal branching occurs due to over-activity of the FGF signaling pathway Over-expression of spry results in an inhibition of FGF mediated tracheal branching. Spry expression was shown to be dependent upon FGF signaling, thus FGF induces expression of its own antagonist. Spry has also been shown to antagonize other receptor tyrosine kinase (RTK) pathways in Drosophila including the EGF receptor pathway (1). Three human and four mouse genes have been identified that have sequence similarity to Drosophila Spry (3,4). Recent investigations in the mouse and the chick embryo indicate that EGF regulates the expression of spry genes in a variety of tissues. Over-expression of spry in the chick limb bud results in reduced wing outgrowth which is consistent with a reduction in FGF signaling from the apical ectodermal ridge (4). Data from this laboratory revealed that spry 2 is expressed in endothelial cells and vascular smooth muscle cells in response to FGF or serum. This suggests that spry acts in the mammalian vasculature in a manner similar to the Drosophila tracheal system. We hypothesize that spry may play a role in the branching morphogenesis of the vascular system and that this regulatory pathway may become disrupted during vascular remodeling or angiogenesis. To address this we propose the following specific aims: 1) determine the mechanism by which spry inhibits FGF signaling in the vasculature using a variety of in vitro and in approaches and 2) to determine the phenotypic and molecular consequences of targeted over-expression of spry in the vasculature of transgenic animals. These studies may provide new insights in vascular homeostasis as well as a potential site for therapeutic intervention of vascular disease.
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